I would think there would be a small gain in mpg, but not a significant amount. It takes so many joules of energy to move your car "X" distance, whether that is via the rear wheels, or the front wheels is irrelevant. It is the conservation of energy. Disengaging the front wheels does not reduce the amount of energy required to move your vehicle, but I would say the amount of wear and tear on CVs and whatnot would make them well worth it.
As far as erotic services go...I believe you will still be able to advertise your personal time for some amount of roses....![]()